Accordingly the conventional procedure for drilling and completing a well requires many hundreds of screw thread connections to be made. Making up of these connections at the drilling floor is a time consuming procedure and it requires use of carefully machined well tubulars.
A significant problem with the conventional screw thread connections is that if the wall strength at the location of the joint needs to be similar to that of the rest of the tubular the accumulated wall thickness of the pipes at the Location of the joint needs to be significantly more than that of the rest of the pipe. The presence of such bulbs at the locations of the joints and the requirement that the string of tubulars must be lowered into the well leads to the requirement of a significant spacing between the external pipe wall and the internal wall of a surrounding tubular such as a casing or the borehole wall. For conventional production tubings with male screw thread joints having a larger outer diameter than the rest of the tubing operational requirements normally require that the minimum spacing between the casing and nominal tubing size is at least 2 cm at each side of the tubing wall. This requirement leads to a significant unused annular space around the production tubing and thus to a reduced production capacity of the well or an increased well size.
It is observed that it is known in the art to use well tubulars with internally and externally flush screw thread connections. A disadvantage of these connections is that they form weak spots with a significantly lower strength than the rest of the pipe.
Safety requirements and explosion hazards at oil or gas wellheads limit the possibility of welding operations. It is known from European patent specification 396204 to safely weld well tubulars together using a friction welding technique where a ring is rotated at high speed while the tubing ends are pressed onto the ring. Disadvantages of the known friction welding technique are, however, that it is difficult to accurately control the temperature and that the ring and pipe ends will often deform which leads to irregular inner and outer surfaces at the location of the joint. The joint therefore often has to brought into a required regular shape by pulling an expansion mandrel therethrough and/or by machining away any wire edges and other irregularities.